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Rosetta visits mysterious asteroid to unlock its secrets

Posted: July 10, 2010
Updated @ 7 p.m. EDT

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Europe's Rosetta spacecraft flew less than 2,000 miles from asteroid Lutetia Saturday, snapping pictures of the new world and collecting bonus science on a primitive relic of the solar system.

Saturn and its rings provide a dramatic backdrop to asteroid Lutetia. Credit: ESA/Max Planck Institute for Solar System Research for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA
Lutetia was unknown before the flyby, and scientists hoped Rosetta would refine estimates of its size, chemical composition and origin.

Rosetta took more than 400 pictures of Lutetia, and the craft's other instruments were programmed to map the asteroid's chemical make-up, probe its interior and search for an atmosphere.

The first results from the flyby were from Rosetta's OSIRIS camera.

Check out a gallery of images from Saturday's encounter.

"I'm startled by the images," said Rita Schulz, Rosetta's project scientist at the European Space Agency. "These are fantastic and exciting pictures, and we should not forget that these are just a few images of all the data that will come down from this instrument, the OSIRIS camera, and this is just one of the many instruments that actually measured during this flyby."

Saturday's encounter made Lutetia the largest asteroid ever studied up-close by a spacecraft.

The sharpest pictures of Lutetia appear to show grooves, landslides and boulder fields, according to Holger Sierks, OSIRIS principal investigator at the Max Planck Institute for Solar System Research in Lindau, Germany.

"It's a new world discovered by Rosetta, and it will keep scientists busy for years," Sierks said.

Engineers inside the European Space Operations Center in Germany confirmed the flyby went as planned at 1610 GMT (12:10 p.m. EDT) Earth time. It took more than 25 minutes for radio signals to travel across the solar system from Rosetta, meaning the closest approach actually occurred at 1544 GMT (11:44 a.m. EDT).

Rosetta was aiming for a point 1,965 miles from Lutetia and a relative velocity of 33,500 mph. The flyby occurred more than 280 million miles from Earth.

ESA unveiled the first high-resolution images of Lutetia in a ceremony at 2100 GMT (5 p.m. EDT).

Flight planners added the Lutetia flyby to Rosetta's $1.2 billion mission as an opportunity to gather ancillary science as the probe flies toward comet Churyuomov-Gerasimenko. Rosetta will orbit the periodic comet for more than a year, observing the icy body from orbit and dropping a small lander to its surface.

"This is just the beginning," said David Southwood, ESA's director of science and robotic exploration. "If this is the way the thing begins, try and imagine how it's going to end. It is an historic day, Europe, once again, proving that it can do major steps in solar system exploration."

Rosetta also visited asteroid Steins in 2008, but the much larger Lutetia offered a chance for ground-breaking science.

These pictures of asteroid Lutetia were taken approximately two minutes before closest approach. Credit: Photo credit: ESA/Max Planck Institute for Solar System Research for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA
"Of the two asteroid flybys that we were able to sneak in on the way to the comet, Lutetia has always been our main asteroid target because we believe this will provide us with the most precious information about how the planets have formed, and what the status of the material was like during the period of planet formation," Schulz said before the flyby.

Despite its maximum diameter of about 80 miles, Lutetia's exact shape and mineral make-up was still a mystery to scientists before Rosetta's visit. The probe confirmed Lutetia has a slightly elongated shape, but it will be several more weeks before scientists know its chemical composition.

Lutetia was discovered in 1852, but the best pictures of the asteroid from telescopes on Earth and in space only show a pixelated object.

The best guess is Lutetia is a C-type asteroid, meaning it has stayed relatively untouched through most of the violent 4.6-billion-year history of the solar system.

C-type asteroids are dark and rich in carbon and organic molecules. Scientists believe they are leftover relics from the formation of the solar system.

"If it does turn out to be a C-type, which we all hope, then we have a large object which is rather pristine showing us what the solar system was like shortly after the planets formed," said Rita Schulz, Rosetta's project scientist, before the Saturday's flyby.

But some measurements suggest Lutetia could harbor metals, a signature of an M-type asteroid. Schulz said metallic M-type asteroids formed from rock from the interior of a larger body after massive collisions fractured the parent object.

"It can't be, at the same time, a C-type and an M-type asteroid because they are so different that it is not possible," Schulz said. "This is a riddle that we can solve only by visiting this object because the indications from all the observations we have right now are not conclusive enough that anyone would dare to say this is for sure a C-type asteroid."

Rosetta's task Saturday was to shed light on these fundamental questions.

The spacecraft was supposed to collect visible images of Lutetia, map its surface with mineral-sniffing spectrometers, look for a thin atmosphere, and study temperature variations on the asteroid. Rosetta was also expected to determine Lutetia's density and search for a weak magnetic field.

"If it does turn out to be a C-type, which we all hope, then we have a large object which is rather pristine showing us what the solar system was like shortly after the planets formed," Schulz said.

Rosetta's OSIRIS camera took this farewell image of Lutetia at the end of the flyby. Credit: ESA/Max Planck Institute for Solar System Research for OSIRIS Team MPS/UPD/LAM/IAA/RSSD/INTA/UPM/DASP/IDA
The Lutetia flyby was the final waypoint for Rosetta on the 10-year voyage from Earth to Churyuomov-Gerasimenko.

Since its launch in 2004, Rosetta completed four gravity assists to bend the robot's trajectory toward the comet, including three flybys of Earth and a single visit to Mars.

Gerhard Schwehm, Rosetta's mission manager, said teams will spend the next year preparing to put the 6,261-pound spacecraft into hibernation, a power-saving deep sleep scheduled to last two-and-a-half years.

Controllers will power up all of Rosetta's science instruments later this year to ensure they are healthy before hibernation. Some payloads will also receive software updates, according to Schwehm.

"Hibernating a spacecraft for something like two years is a big deal, and you want to make sure you can wake it up again," Schulz said.

As Rosetta spirals into the outer solar system, ground teams will test its large solar panels in low-intensity mode, a special feature that boosts the efficiency of the probe's power-production system as the arrays collect less sunlight.

Rosetta's sensitive solar wings stretch 105 feet tip to tip, providing a large collecting area for solar cells to convert weaker sunlight into electricity.

Engineers also plan a lengthy deep space maneuver in January to put Rosetta on course for Churyuomov-Gerasimenko. Four of Rosetta's thrusters will change the probe's velocity by roughtly 1,767 mph, according to Schwehm.

"We will have afterwards a very quiet period for the spacecraft that we can monitor all the subsystems to be sure everything is OK and working properly, and in June we will put Rosetta in hibernation," Schwehm told Spaceflight Now.

Artist's concept of Rosetta approaching Churyuomov-Gerasimenko in 2014. Credit: ESA
Officials do not plan to wake up Rosetta during hibernation, partially because the spacecraft will not have enough electricity to power all of its systems at one time.

"We can't do a lot as we wouldn't have enough power to run the spacecraft with all systems on to correct possible problems," Schwehm said.

Another large thruster burn is on tap for the spring of 2014, just after Rosetta awakes from hibernation in the last few months before arrival at the comet.

Rosetta should arrive close to Churyuomov-Gerasimenko in May 2014 and enter orbit around the 2.5-mile wide comet nucleus as it approaches the sun.

The spacecraft will deploy Germany's Philae lander to the surface of the comet in early Novemer 2014, where it will send back images and data for up to a week.

Rosetta will stay with the comet through 2015 as it passes near the sun, mapping its dark surface and observing its changing characteristics as it heats up and outgasses volatile compounds like water ice.